Antenna diversity allows a station to reduce fast fading effects and therefore to improve reception by making use of multiple propagation paths. Diversity can be obtained by locating antennas in different spaces or by receiving signals of different polarization on two antennas located in the same general space.
Spatial diversity is usually achieved by placing one antenna away from another antenna horizontally or vertically. Horizontal diversity leads to good diversity gain, typically around 3 to 5 dB. To achieve horizontal spacial diversity, separation of several feet at PCS frequencies, such as 1.9 GHz, are required, leading to large antenna setup on a base station tower. A smaller setup can be achieved with vertical diversity. Unfortunately, in order to achieve sufficient decorrelation of signals, vertically spaced antennas must be placed further apart. This generally results in a stronger received signal from higher antennas, reducing diversity gain to typically less than 2 dB.
Polarization diversity consists of co-locating two antennas with different polarizations. The two polarization angles should be orthogonal to achieve sufficient decorrelation of the received signals. Possible configurations include horizontal/vertical (HV) polarization and slant .+-.45.degree. polarization. HV polarization suffers from different propagation characteristics in the different polarization directions, with horizontal signals propagating worse than vertical. Slant .+-.45.degree. polarization is better balanced but has the major drawback that, when also used to transmit, the antenna is not as efficient as a vertical polarization.
What is required are antennas that couple the benefits of both spacial and polarization diversity.